I had heard most of what he wrote before, but never synthesized into a single, coherent description of the experience of being introverted. I kept having "Hey, that's me!" moments while I read - and many "Hey! That's [insert name here]!" moments too. I did some more exploring, both on the web and in my head, and discovered that 'introvert' and 'extrovert' are not exactly the right terms for me; in the same situation, I may act as either.
These terms are based on Jung's definitions of Introvert and Extrovert, which are a little different from the common use. From this perspective, there is no continuum between introversion and extraversion. The terms are not used to indicate sociability; instead, they indicate if a person is oriented to a subjective or objective attitude. Introverts find focus and energy from within. Extroverts find these in their environment.
It turns out there is a third category, for people like me. There is some disagreement about the validity of the term. In my experience, most people have an exclusionary view of personality traits: that we are each one thing, and therefore not another, opposite thing. Some are obviously inappropriate stereotypes (beauty or brains); some are more subtle (empathy or evil (see psychopathy)). I think we can have many traits and modes of operation which we expose based on circumstance.
In the case of intro- vs. extro-, most of us have an obviously dominant mode, and no choice about acting in that mode. In some cases - me for instance - a person does not have a clearly dominant mode. For us, some combination of internal and external factors pushes us to one or the other. Sometimes we can even choose. I'm sure this makes it hard on our friends and families: how do you know what strategy to use when I'll handle a situation in radically different ways based on no discernible factor?
One of the most popular, the Keirsey, divides people along four axes:
Extroversion (E) and Introversion (I)
Intuition (N) and Sensing (S)
Thinking (T) and Feeling (F)
Judgement (J) and Perception (P)
I've taken it many times, from many perspectives. In each case I have only two traits: high T, low F and high N, low S. I plotted it out and found this (each score out of 20):
After all that, you'd think I'd have a really serious payoff for the last paragraph, but I don't.
Edit 2012-12-25 in Poughkeepsie, NY
These are still great resources for pretty much everyone. Also, in the new Google Blogger, the title showed up as 'Human Interest: The Care and Feeding of your Intro". So I wondered what exactly I had posted as my introductory blog post. Is that +Alanis Morissette enough for you? :)